1. Field of the Invention
The present invention generally relates to the field of detecting defects, such as pits, mounds, particles, scratches, stains, crystalline defects, or the like, on epitaxial layers. More specifically, this invention relates to detecting defects on an epitaxial layer by way of a multi-wavelength, optical surface analyzer.
2. Description of Background Art
Coated thin film disks are used in a variety of industries including the semiconductor and the magnetic hard disk industry. An epitaxial growth process may produce a coating on semiconductor wafers. The process consists of growing a layer of a particular composition upon a substrate, which has a similar crystalline structure. Defects can be caused by a thermal or structural mismatch between the epitaxial layer and the substrate material during the growth process. It is important to be able to detect and classify these defects in order to prevent failure of electronic devices which are patterned upon the epitaxial layer and to control the manufacturing process.
Conventional optical surface analysis techniques are available for defect analysis of transparent epitaxial layers; however, significant shortcomings are present. Conventional surface analysis systems operate at a single laser wavelength. Also, some systems use non-laser light (polychromatic) sources for spot thickness measurement only and not a full surface measurement of the defects of an epitaxial layer. These types of systems are used to measure thickness variations only and are not capable of measuring defects. Polychromatic systems are slow and perform only spot measurements on the wafer surface. As a result these systems only examine a small percentage of the wafer surface and would miss the vast majority of small defects.
Single wavelength laser systems are also available; however, they have difficulty detecting all defects on an epitaxial surface since they suffer from nulls in their sensitivity at certain critical points which result from thickness variations in the epitaxial layer. Critical points are common within monochromatic systems due to the presence of interference fringes that appear in the reflectivity images as the thickness of the sample being analyzed changes.
What is needed is an automated method for detecting a variety of defects contained within a grown epitaxial layer independent of the thickness of the epitaxial layer.